Variable pressure hydraulic system

ABSTRACT

A variable pressure hydraulic system in which a motor is operated by a variable displacement pump whose displacement is regulated by a control valve which is sensitive to the pressure at the hydraulic motor to maintain the pump outlet pressure higher than the motor pressure. A normally open solenoidcontrolled valve is movable upon actuation of the solenoid to a closed position in which it blocks communication between the control valve and the hydraulic motor. The electrical circuit for the solenoid valve includes a pressure sensitive switch responsive to the fluid pressure at the hydraulic motor so that when the fluid pressure at the hydraulic motor reaches a predetermined value, the solenoid is activated to close the solenoid-controlled valve and block communication between the control valve and the hydraulic motor. In this manner, the maximum pressure of the system is limited.

United States Patent [19 Bahl 1 3 ,826,090 July 30, 1974 Appl. No.: 382,516

US. Cl 60/445, 60/452, 60/DIG. 2, 91/358 Int. Cl. F 16h 39/46 Field of Search 60/388, 445, 452, 465,.

60/DIG. 2, 420

References Cited UNITED STATES PATENTS l0/l96l Raymond 4l7/2l3 Allen et al. 60/427 Primary Examiner-Edgar W. Geoghegan 5 7] ABSTRACT A variable pressure hydraulic system in which a motor is operated by a variable displacement pump whose displacement is regulated by a control valve which is sensitive to the pressure at the hydraulic motor to maintain the pump outlet pressure higher than the motor pressure. A normally open solenoid-controlled valve is movable upon actuation of the solenoid to a closed position in which it blocks communication between the control valve and the hydraulic motor. The electrical circuit for the solenoid valve includes a pressuresensitive switch responsive to the fluid pressure at the hydraulic motor so that when the fluid, pressure-at the hydraulic motor vreaches a predetermined value, the solenoid is activated to close the solenoidcontrolled valve and block communication between the control valve and the hydraulic motor. In this manner, the maximum pressure of the system is limited.

12 Claims, 2 Drawing Figures PATENIED JUL3 0 I974 SHEET 10F 2 VARIABLE PRESSURE HYDRAULIC SYSTEM BACKGROUND OF THE INVENTION The present invention relates generally to hydraulic systems, and more particularly relates to a closed center, variable pressure hydraulic system of the type including a variable displacement pump where stroke or output is controlled according to the demands of the system.

Closed center hydraulic systems can generally be of two types, constant pressure and the so-called variable pressure, demand compensated systems in which the pump output is controlled according to the demands of the system. In a constant pressure system, the pressure is maintained at the highest pressure requirement of the system and the flow rate is controlled according to the flow requirements of the system. The constant pressure system is under continual strain and is susceptible to leaks due to the continual high pressure.

With a demand compensated variable pressure system, the pump output, both flow and pressure, is conleaks as does a constant pressure system.,For example,

if the system includes a plurality of functions or hydraulic motors having different pressure demands, a predetermined minimum standby pressure is established 1 control valve for an expansible chamber hydraulic motor is often kept open when the motor is tobe fully extended and maintained fully extended. In this instance, the high system pressure is maintained as long as the control valve is open. As shown in copending US. application Ser. No. 245,793 filed on 20 Apr.

, 1972 now US. Pat. No. 3,754,400 issued Aug. 28,

1973 by D. J. Parquet, and assigned to the assignee of the present application, attempts have been made to overcome this problem inherent in demand compen'-.

sated systems by automatically returning the system to the standby pressure when the hydraulic motor has been fully extended. However, if the hydraulic cylinder was loaded to the point where the maximum system pressure was inadequate to fully extend the hydraulic motor, it was still possible to maintain the high system pressure by leaving the valve open.

SUMMARY OF THE INVENTION The principal object of the present invention is to provide a hydraulic system'of the demand compensated type which includes means to return the pump to standby pressure whenever the pressure delivered to a function reaches a predetermined maximum value.

A further object of the present inventionis to provide a demand compensated hydraulic system'in which high system pressures are not maintained for a greater time than absolutely necessary for function control and in which the high system pressure cannot be maintained if the function to be controlled is loaded beyond the system capacity.

Another object of the present invention is to provide a variable pressure and variable volume hydraulic system in which the variable displacement pump is under the control of a first valve which is responsive to the pressure of a hydraulic function within the system to increase the system pressure asthe pressure at the hydraulic function increases, and which includes a second valve responsive to a predetermined maximum pressure at the hydraulic function to close and block communication between the first valve and the motor so that at any time the pressure at the hydraulic function reaches the predetermined maximum value, the system pressure is reduced to a predetermined minimum standby pressure.

The above objects and additional objects and advantages of the present invention will become apparent along with the details of construction of preferred embodiments of the inventionfrom a reading of the following detailed descriptions when taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING In the drawing:

FIG. 1 is a schematic illustration of a hydraulic sys-- tem according to the present invention; and,

FIG. 2 is a schematic illustration of a second and modified hydraulic system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, and particularly to 3 Oct. 1961. Specifically, the pump 10 is of the type which has a plurality of reciprocating pistons projecting into a drive chamber for engagement with a drive cam and in which pressure within the chamber will hold the pistons away from the drive cam so that as pressure within the chamber increases the stroke or output of the pump decreases.

The intakeof the pump 10 is connected to a reservoir 16 bya fluid line 18, and the output of thepump 10 is connected to a first side of the control valve 14 by a fluid line 20. The first side of the control valve 14 is also connected to the reservoir 16 by a fluid line 22. Thesecond side of the control valve 14 is connected to the expansible chamber hydraulic motor 12 by fluid lines 24 and 25. The fluid line 24 is a pressure supply line provided with check valve 26 which prevents reverse flow and the fluid line 25 is an exhaust line provided with a variable restrictor 27 or rate of drop valve. Movementof the control valve 14 to one side of its neutral position interconnects the pressure supply line 24 with the fluid line 20, and movement of the control bore 54 and acts on the piston 52.

valve'14 to the opposite side of its neutral position inte'rconnects the pressureexhaust line 25 with the reservoir 16 through the fluid line 22. i

For exemplary purposes, the expansible chamber hydraulic motor 12 is shown as controlling a conventional rockshaft 28 and rock arm 30 of an agricultural tractor, and the control valve 14 is a servo valve responsive to the position of the selectively settable hand lever 32 through linkage 34 and the position of the rockshaft 28 through linkage 36. The linkages 34 and-36 are typical of those found in an agricultural tractor and are generally constructed so that movement of the lever 32 moves the control valve 14 from the neutral position until the rockshaft has reached a position determined by the setting of the lever 32, at which time the linkage 36 closes the valve. However, when the lever 32 is set for the maximum up position of the rockshaft 28 and rock arm 30, the-linkage 36 does not return the valve 14 to the neutral position but leavesthe valve in an open position. p

The pressure within the drive chamber of the pump is controlled by a valve '38 which is connected to the fluid line 20 or the pump output by-fluid line 40 and to .the pump drive chamber by fluid lines 42 and 44', the

fluid line 42 also being connected to the pump intake line through a-restrictor 46. The valve- 38 is normally biased to a closed position by a spring 48 and is responsive to the fluid pressure within the fluid line 40 through a pilot line 50 to open whenever the pressure in the fluid line 40 is great enough to overcome the forcevof the spring 48.

A piston 52 slidably mounted in a cylinder or bore 54 acts against the spring 48 and is responsive to fluid pressure within the bore 54 to increase the force which the spring 48 applies against the valve 38. A pilot line 56 interconnects the bore 54 and the fluid line 24 be-- tween the valve 14 and check valve26 so that the fluid pressure supplied to the motor 12 is transferred to the A solenoid-controlled valve 58 is interposed in the pilot line 56 and is normally biased to an open position by a spring 60. The valve 58 is movable to a closed position in which it blocks communication between the fluid line 24 and bore 54, and when in the closed position also connects the bore 54 and the pilot line 56 to the reservoir 16 through a fluidline 62. The electricalcircuit for the solenoid 64 of the solenoid-controlled valve 58 includes an electrical'source 66 and a pressure sensitive switch indicated generally at 68. The switch 68 includes a fixed contact 70 mounted on any suitable support 72 through an insulator 74. The switch 68 also includes a movable contact 76 mounted on the end of a piston rod 78 through an insulator 80. The piston rod 78 is carried by a piston 82 slidably mounted in a cylinder or bore 84. The piston 82 is nonnally biased to one extreme end of the bore 84 by a spring 86 to normally maintain the contacts 70 and 76 separated.

A fluid line 88 establishes communication between the hydraulic motor 12 and bore 84 so that'the fluid pressure at the hydraulic motpr is also appliedto the piston 82. With this construction, the switch 68 will close to activate the solenoid 64 and move the valve 58 V to the closed position as soon as the pressure at the hydraulic motor 12 reaches a predetermined maximum value at which value the spring 86 is compressed and the rod 78 extended. When the valve 58 is moved to the closed position, the fluid pressure in the bore 54 and pilot line 56 will be exhausted to the reservoir 16 through line 62.

Although not illustrated, the hydraulic system may include additional hydraulic motors and their associated motor control valves, and the additional hydraulic motors may or may not be connected to the bore 54 through pilot lines depending upon whether or not they require a pressure greater thanthe predetermined minimum standby pressure established by the spring 48.

' The operation of the above-described hydraulic system is as follows. Assuming the valve 14 is closed, the output of the pump 10 will increase to the predetermined minimum standby pressure established by the spring 48. At this time, the valve 38 will shift to an open position so that the output pressure of the pump 10 is routed through the valve 38 and .the fluid lines 42 and 44 to the drive chamber of the pump 10. The pressure within the drive chamber of the pump 10 holds the reciprocating pistons off the'drive cam so that the output of the pump 10 is maintained at the predetermined minimum standby pressure established by the spring 48. If the valve 14 is moved from its neutral position to interconnect the hydraulic motor 12 and the pump 10, the fluid pressure at the motor 12 is routed through the pilot line56 to the bore 54 and acts on the piston 52 to increase the pressure applied to the valve 38 by the spring 48 so that the valve 38 shifts to its closed position. The fluid pressure within the drive chamber of the pump 10 exhausts through the restrictor 46 so that the output of the pump 10 increases.

If the valve 14 is closed before the valve 58 is closed by excessive pressure, the fluid pressure in the pilot line 56 and bore 54 will be exhausted through a restricted orifice 90 so that the pressure applied to the valve 38 is reduced and the pump output will be reduced to the predetermined minimumstandby pressure established by the spring 48. If desired, the orifice 90 can be omitted since there will be someleakage across the piston 52 and this leakage will reduce the pressure in the bore 54' and pilot line 56.

If the pressure at the motor 12 reaches the value necessary to move thepiston 82 and piston rod 78 a sufficient distance to close the contacts 72 and 76'before the valve 14 is closed, the valve 58 will move to the closed position so that the bore 54 and pilot line 56 are exhausted to the reservoir 16 through the fluid line 62.

'Thisimmediately reduces the pump output pressure to the predetermined minimum standby pressure. The system will remain in this condition until such time as the pressure is relieved from the motor 12 by manual movement of the valve 14 to the opposite side of the neutral position or by operation of an additional function not illustrated.

A modified hydraulic system according to the present invention is illustrated in FIG. 2 and includes a variable displacement pump 110, a function in the form of an expansible chamber hydraulic motor 112 or an extensible and retractable hydraulic cylinder 112, and a control valve 114. The displacement of the pump is controlled by a pressure chamber formed by a piston 194 within a cylinder 192. The piston 194 is connected through a piston rod 198 to a pump control member such as a swashplate. A spring 196 acts on the piston 194 to bias the piston 194 and rod 198 to a maximum displacement position. The displacement of the pump 110 is reduced by the introduction of fluid under pressure into the pressure chamber formed by the piston 194 and cylinder 192 so that the piston 194 and rod 198 are moved against the biasing force of the spring 196.

The intake of the pump 110 is connected to a reservoir 116 by a fluid line 118, and the output of the pump 110 is connected to a first side of the control valve 114 by a fluid line 120. The first side of the control valve 114 is also connected to the reservoir 116 by fluid line 122. The second side of the control .valve 114 is connected to the expansible chamber hydraulic motor 112 by fluid lines 124 and 125. The fluid line 124 is a pressure supply line provided with a check valve 126 which prevents reverse flow and the fluid line 125 is an exhaust line provided with a variable restrictor 127 or rate of drop valve. Movement of the control valve 114 to one side of its neutral position interconnects the pressure supply line 124 with the fluid line 120, and movement of the control valve 114 to the opposite side of its neutral position interconnects the pressure exhaust line 125 with the reservoir 116 through the fluid line 122.

For exemplary purposes, the expansible chamber hydraulic motor 112 is shown as controlling a conventional rockshaft 128 and rock arm 1300f an agricultural tractor, and the control valve 114 is a servo valve responsive to the position of the selectively settable hand lever 132 through linkage 134 and the position of the rockshaft 128 through linkage 136. The linkages 134 and 136 are typical of those found in an agricultural tractor and are generally constructed so that movement of the lever 132 moves the control valve 114 from the neutral position until the rockshaft has reached a position determined by the setting of the lever 132, at which time the linkage 136 closes the valve. However, when the lever 132 is set for the maximum up position of the rockshaft 128 and rock arm 130, the linkage 136 does not return the valve 114 to tne neutral position but leaves the valve in an open position.

The pressurewithin the pressure chamber formed by the piston 194 and cylinder 192 is controlled by a valve 138 which is connected to the fluid line 120 or the pump output by a fluid line 140 and to the pump pressure chamber by fluid lines 142 and 144, the fluid line 142 also being connected to the pump intake line through a restrictor 146. The valve 138 is normally biased to a closed position by a spring 148 and is responsive to the fluid pressure within the fluid line 140 through a pilot line 150 to open whenever the pressure in the fluid line 140 is great enough to overcome the force of the spring 148.

A piston 152 slidably mounted in a cylinder or bore 154 acts against the spring 148 and is responsive to fluid pressure'within the bore 154 to increase the force which the spring 148 applies against the valve 138. A pilot line 156 interconnects thebore 154 and the fluid line 124 between the check valve 126 and the motor 112 so that the fluid pressure supplied to the motor 112 is transferred to the bore 154 and acts on the piston 152.

pilot line 156 and is normally biased to an open position by a spring 160. The valve 158 is movable to a closed position in which it blocks communication between the fluid line 124 and bore 154, and when in the closed position also connects the bore 154 and the pilot line 156 to the reservoir 116 through a fluid line 162.

.A solenoid-controlled valve 158 is interposed in the The electrical circuit for the solenoid 164 of the solenoid-controlled valve 158 includes an electrical source 166 and a pressure sensitive switch indicated generally at 168. The switch 168 includes a fixed contact 170 mounted on any suitable support 172 through an insulator 174. The switch 168 also includes a movable contact 176 mounted on the end of a piston rod 178 through an insulator 180. The piston rod 178 is carried by a piston 182 slidably mounted in a cylinder or bore 184. The piston 182 is normally biased to one extreme end of the bore 184 by a spring 186 to'normally maintain the contacts 170 and 176 separated.

A fluid line 188 establishes communication between the hydraulic motor 112 and bore 184 so that the fluid pressure at the hydraulic motor is also applied to the piston 182. With this construction, the switch 168 will close to activate thesolenoid 164 and move-the valve 158 to the closed position as soon as the pressure at the hydraulic motor 112 reaches a predetermined maximum value at which value the spring 186 iscompressed and the rod 178 extended. When the valve 158 is moved to the closed position, the fluid pressure in the bore 154 and pilot line 156 will be exhausted to the reservoir 116 through line 162.

Although not illustrated, the hydraulic system may include additional hydraulic motors and their associated motor control valves, and the additional hydraulic motors may or may not be connected to the'bore 154 through pilot lines depending upon whether or not they require a pressure greater than the predetermined minimum standby pressure established by the spring 148.

The operation of theabove-described hydraulic systemis as follows. Assuming the valve 114 is closed, the output of the pump will increase to the predetermined minimum standby pressure established by the spring 148. At this time, the valve 138 will shift to an open position so that the output pressure of the pump 110 is routed through the valve 138 and the fluid lines 142 and'144 to the pressure chamber formed by the piston 194 and cylinder 192. The pressure within the pressure chamber moves the piston 194 and rod 198 upwardly to adjust the pump control member to decrease the'stroke of the pump so that the pump 110 is maintained at the predetermined minimum standby pressure established by the spring 148. If the valve 114 is moved from its neutral position to interconnect the hydraulic motor 112 and the pump 110, the fluidpressure at the motor 112 is routed through the pilot line 156-to the bore 154 and acts on the piston 152 to increase .the pressure applied to the valve 138 by the spring 148 so that the valve 138 shifts to its closed position. The fluid pressure within the pressure chamber formed by the piston 194 and cylinder 192 exhausts through the restrictor 146 so that the output of the pump 110 increases. 7

If the valve 114 is closed before the valve 158 is closed'by excessive pressure, the solenoid-controlled valve 158 and switch 168 will have no effect on the system, and the pilot line 156, bore 154 and piston 152 will operate to provide the pressure and flow necessary to control the motor 112. However, should the pressure at the motor 112 reach or exceed the pressure required to close the switch 168 before the valve 114 is'closed because of excessive load on the lift arm or because the motor 112 has reached the end of its stroke, the valve 158 will move to its closed position to limit the pressure delivered to the motor 112. As soon as the valve 158 is moved to its closed position, the pressure within the bore 154 and pilot line 156 is exhausted to the reservoir 116 through the fluid line 162 so that the pump 110 returns to its standby pressure. This system will-remain in this condition until such time as the valve 114 is moved to the opposite side of the neutral position so that the pressure is exhausted from the motor 112.

The hydraulic system illustrated in FIG. 2 differs from that illustrated in FIG. 1 not only in the specific construction of the pump, but also in operation as long as the system pressure does not meet or exceed the pressure necessary to close the switch 168. In the FIG. 1 embodiment, the system always returns to the predetermined minimum standby pressure as soon as the valve 14 is moved to its closed position. In the FIG. 2 embodiment, the pump 110 will have output pressure at a predetermined value above the pressure atthe motor 112 as long as the pressure at the motor 112 does not meet or exceed the pressure necessary to close the switch 168. Thus, the hydraulic system illustrated in FIG. 2 can be maintained at higher pressures for a greater time than can the system illustrated in FIG. 1. However, experience has shown that the control of the hydraulic motor for a conventional rockshaft and rock arm of an agricultural tractor requires only a small percentage of the total usage of a hydraulic system, so that for the greatest-percentage of the time, the hydraulic system will be at its normal standby pressure for the control of other functions. Thus, the system illustrated in FIG. 2 will provide a higher pressure for the control of the rockshaft 128, will limit the system pressure so that safe pressure levels will not be exceeded, and will return the system to standby pressure as. soon as the connected to' and for moving thenormally open valve means responsive to pressure at the hydraulic motor to move the valve means to the closed position when the fluid pressure at the hydraulic motor'reaches a predetermined value to thereby limitthe maximum pump output pressure.

2. A variable pressure hydraulic system as set forth in claim 1 wherein the valve control means includes a solenoid operatively associated with the valve means, an electrical circuit for the solenoid, and a pressure sensitive switch in the electrical circuit.

3. A variable pressure hydraulic system as set forth in claim 2 wherein the solenoid moves the valve means to the closed position when activated, the pressure sensitive switch is normally biased to the open position and closes to complete the electrical circuit when the fluid pressure at the hydraulic motor reaches the predetermined value.

- 4. A variable pressure hydraulic system comprising: a variable displacement pump having output control means including a pressure chamber operative to vary pump output according to changes in pressure in the chamber; the pump havingan outlet; a hydraulic motor; fluid line means connecting the hydraulic motor to the pump outlet; a pump control valve having a movable valve member; means operable by pump outlet pressure to move the valve member in a direction to maximum pressure is met or exceeded so that only that portion of the system between the valve 114 and valve 158 can be maintained at'the maximum system pressure for any length of time.

Having thus described and illustrated two specific embodiments of the invention, various modifications within the spirit and scope of the invention will become apparent to those skilled in the art and can be made without departing from the underlying principles of the invention. Therefore, the invention should not be limited to the specific illustrations and description, but only by the following claims.

I claim:

1. A variable pressure hydraulic system comprising: a variable displacement pump having output control means including a pressure chamber operative to reduce pump output as pressure in the chamber increases; the pump having an outlet; a hydraulic motor connected to the pump outlet; a pump control valve having a movable valve. member responsive to pump outlet pressure to move in a direction to operatively interconnect the pump outlet and the pressure chamber; yieldable force-applying means biasing the valve member in an opposite direction to establish a minimum pump output pressure; means normally responsive to fluid pressure delivered to the hydraulic motor to in crease the pressure level necessary to move the valve member to operatively interconnect the pump outlet and the pressure chamber; normally open valve means movable to a closed position in which it blocks fluid pressure from the fluid pressure responsive means; and pressure responsive valve control means operatively operatively interconnect the pump outlet and the pressure chamber; first means operable to bias the valve member in the opposite direction to establish a minimum pump outlet pressure; a pilot line connected to I the fluid line means to be pressurized in accordance with the pressure at the hydraulic motor; second means responsive to fluid pressure in the pilot line operable to apply a biasing force on the valve member to increase the pressure level at said pump outlet necessary to move the valve member to operatively interconnect the pump outlet and the pressure chamber; a normally open valve means interposed in the pilot line movable to a closed position in which it blocks fluid communication between the second means and the fluid line means; and control means operatively associated with the valve means responsive to a predetermined fluid pressure at the hydraulic motor to move the valve means to the closed position.

5. A variable pressure hydraulic system as set forth in claim 4 wherein the control means includes a solenoid connected to and for controlling the movement of the valve means, an electrical circuit for the solenoid, and a pressure sensitive switch in the electrical circuit.

6. A variable pressure hydraulic system as set forth in claim 5 wherein the pressure sensitive switch includes a movable piston carrying a switch contact and having one end in fluid communication with the hydraulic motor, and spring means acting against the opposite end of the piston to normally move the piston in a direction to open the switch.

7. A hydraulic system comprising: a variable displacement pump having output control means including a pressure chamber operative to vary pump output according to changes in pressure in the chamber; a fluid motor; a high pressure line connected between the pump and motor; a motor controlvalve interposed in the high pressure line for selectively opening and closing said high pressure line to the motor; fluid passage means between the high pressure line upstream of the motor control valve and the pressure chamber; a spring-loaded normally closed valve in the fluid passage means normally blocking communication between the high pressure line and pressure chamber and responsive to a predetermined pressure in the high pressure line to open and establish communication between the high pressure line and the pressure chamber; a pilot line connected between the high pressure line downstream of the control valve and the spring-loaded valve to apply the pressure in the high pressure line downstream of the control valve to the spring-loaded valve to increase the closing force on the spring-loaded valve; normally open valve means in the pilot line movable to a closed position in which it blocks communication between the high pressure line downstream of the control valve and the spring-loaded valve, and pressure sensitive valve control means operatively connected to and for moving the valve means responsive to pressure at the fluid motor to move the'valve means to the closed position when the fluid pressure at the fluid motor reaches a predetermined value.

8. A hydraulic system as set forth in claim 7 wherein the valve control means includes a solenoid operatively associated with the valve means, an electrical circuit for the solenoid, and a pressure sensitive switch in the electrical circuit.

. 9. A hydraulic system as set forth in claim 8 wherein a check valve is interposed in the high pressure line between the fluid motor and motor control valve, an exhaust line interconnects the fluid motor and motor control valve, the pilot line is connected to the high pressure line between the check valve and motor control valve, and means are provided to afford a controlled bleed of fluid from the pilot line.

10. A variable pressure hydraulic system comprising: a variable displacement pump having stroke control means including a pressure chamber operative to decrease the pump stroke as the pressure in the chamber increases; the pump having an outlet; a hydraulic motor; fluid line means interconnecting the pump outlet and the motor; a flow control valve interposed in the fluid line means for selectively opening and closing the fluid line means to the motor; apump control valve having a movable valve-member responsive to pressure at the pump outlet to move in a direction to operatively interconnect the pump outlet and the pressure chamber; yieldable force-applying means biasing'the valve member in an opposite direction to establish a minimum pump outlet pressure; fluid pressure responsive means acting on the pump control valve to increase the pressure level at the pump outlet necessary to move the valve member to operatively interconnect the pump outlet with the pressure chamber; pilot line means interconnecting the pressure responsive means with the fluid line means between the hydraulic motor and the flow control valve; normally open solenoidcontrolled valve means interposed in the pilot line means and movable to a closed position in which it blocks fluid pressure from the pressure responsive means upon actuation of the solenoid; and normally open pressure responsive switch means responsive to a predetermined fluid pressure level at the hydraulic motor to close and complete an electrical circuit for the solenoid to activate the solenoid.

11. A variable pressure hydraulic system as set forth in claim 10 wherein the valve means interposed in the fluid line means connects the pressure. responsive means with the fluid reservoir when in the closed position.

12. 'A hydraulic system as set forth in claim 11 wherein a check valve is interposed in the high pressure 

1. A variable pressure hydraulic system comprising: a variable displacement pump having output control means including a pressure chamber operative to reduce pump output as pressure in the chamber increases; the pump having an outlet; a hydraulic motor connected to the pump outlet; a pump control valve having a movable valve member responsive to pump outlet pressure to move in a direction to operatively interconnect the pump outlet and the pressure chamber; yieldable force-applying means biasing the valve member in an opposite direction to establish a minimum pump output pressure; means normally responsive to fluid pressure delivered to the hydraulic motor to increase the pressure level necessary to move the valve member to operatively interconnect the pump outlet and the pressure chamber; normally open valve means movable to a closed position in which it blocks fluid pressure from the fluid pressure responsive means; and pressure responsive valve control means operatively connected to and for moving the normally open valve means responsive to pressure at the hydraulic motor to move the valve means to the closed position when the fluid pressure at the hydraulic motor reaches a predetermined value to thereby limit the maximum pump output pressure.
 2. A variable pressure hydraulic system as set forth in claim 1 wherein the valve control means includes a solenoid operatively associated with the valve means, an electrical circuit for the solenoid, and a pressure sensitive switch in the electrical circuit.
 3. A variable pressure hydraulic system as set forth in claim 2 wherein the solenoid moves the valve means to the closed position when activated, the pressure sensitive switch is normally biased to the open position and closes to complete the electrical circuit when the fluid pressure at the hydraulic motor reaches the predetermined value.
 4. A variable pressure hydraulic system comprising: a variable displacement pump having output control means including a pressure chamber operative to vary pump output according to changes in pressure in the chamber; the pump having an outlet; a hydraulic motor; fluid line means connecting the hydraulic motor to the pump outlet; a pump control valve having a movable valve member; means operable by pump outlet pressure to move the valve member in a direction to operatively interconnect the pump outlet and the pressure chamber; first means operable to bias the valve member in the opposite direction to establish a minimum pump outlet pressure; a pilot line connected to the fluid line means to be pressurized in accordance with the pressure at the hydraulic motor; second means responsive to fluid pressure in the pilot line operable to apply a biasing force on the valve member to increase the pressure level at said pump outlet necessary to move the valve member to operatively interconnect the pump outlet and the pressure chamber; a normally open valve means interposed in the pilot line movable to a closed position in which it blocks fluid communication between the second means and the fluid line means; and control means operatively associated with the valve means responsive to a predetermined fluid pressure at the hydraulic motor to move the valve means to the closed position.
 5. A variable pressure hydraulic system as set forth in claim 4 wherein the control means includes a solenoid connected to and for controlling the movement of the valve means, an electrical circuit for the solenoid, and a pressure sensitive switch in the electrical circuit.
 6. A variable pressure hydraulic system as set forth in claim 5 wherein the pressure sensitive switch includes a movable piston carrying a switch contact and having one end in fluid communication with the hydraulic motor, and spring means acting against the opposite end of the piston to normally move the pistOn in a direction to open the switch.
 7. A hydraulic system comprising: a variable displacement pump having output control means including a pressure chamber operative to vary pump output according to changes in pressure in the chamber; a fluid motor; a high pressure line connected between the pump and motor; a motor control valve interposed in the high pressure line for selectively opening and closing said high pressure line to the motor; fluid passage means between the high pressure line upstream of the motor control valve and the pressure chamber; a spring-loaded normally closed valve in the fluid passage means normally blocking communication between the high pressure line and pressure chamber and responsive to a predetermined pressure in the high pressure line to open and establish communication between the high pressure line and the pressure chamber; a pilot line connected between the high pressure line downstream of the control valve and the spring-loaded valve to apply the pressure in the high pressure line downstream of the control valve to the spring-loaded valve to increase the closing force on the spring-loaded valve; normally open valve means in the pilot line movable to a closed position in which it blocks communication between the high pressure line downstream of the control valve and the spring-loaded valve, and pressure sensitive valve control means operatively connected to and for moving the valve means responsive to pressure at the fluid motor to move the valve means to the closed position when the fluid pressure at the fluid motor reaches a predetermined value.
 8. A hydraulic system as set forth in claim 7 wherein the valve control means includes a solenoid operatively associated with the valve means, an electrical circuit for the solenoid, and a pressure sensitive switch in the electrical circuit.
 9. A hydraulic system as set forth in claim 8 wherein a check valve is interposed in the high pressure line between the fluid motor and motor control valve, an exhaust line interconnects the fluid motor and motor control valve, the pilot line is connected to the high pressure line between the check valve and motor control valve, and means are provided to afford a controlled bleed of fluid from the pilot line.
 10. A variable pressure hydraulic system comprising: a variable displacement pump having stroke control means including a pressure chamber operative to decrease the pump stroke as the pressure in the chamber increases; the pump having an outlet; a hydraulic motor; fluid line means interconnecting the pump outlet and the motor; a flow control valve interposed in the fluid line means for selectively opening and closing the fluid line means to the motor; a pump control valve having a movable valve member responsive to pressure at the pump outlet to move in a direction to operatively interconnect the pump outlet and the pressure chamber; yieldable force-applying means biasing the valve member in an opposite direction to establish a minimum pump outlet pressure; fluid pressure responsive means acting on the pump control valve to increase the pressure level at the pump outlet necessary to move the valve member to operatively interconnect the pump outlet with the pressure chamber; pilot line means interconnecting the pressure responsive means with the fluid line means between the hydraulic motor and the flow control valve; normally open solenoidcontrolled valve means interposed in the pilot line means and movable to a closed position in which it blocks fluid pressure from the pressure responsive means upon actuation of the solenoid; and normally open pressure responsive switch means responsive to a predetermined fluid pressure level at the hydraulic motor to close and complete an electrical circuit for the solenoid to activate the solenoid.
 11. A variable pressure hydraulic system as set forth in claim 10 wherein the valve means interposed in the fluid line means connects the pressure responsive means with the fluid reservoir when in the closed position.
 12. A hydraulic system as set forth in claim 11 wherein a check valve is interposed in the high pressure line between the fluid motor and motor control valve, an exhaust line interconnects the fluid motor and motor control valve, the pilot line is connected to the high pressure line between the check valve and motor control valve, and means are provided to afford a controlled bleed of fluid from the pilot line. 